1. Field of the Invention
This invention relates to an engine controller for feeding-back and controlling an air fuel ratio of an internal combustion engine, and particularly relates to an improvement of an abnormality diagnosing device built-in and used in the engine controller, and detecting a breakdown and a wiring abnormality of an exhaust gas sensor externally connected to detect the air fuel ratio and annunciating the abnormality.
2. Background Art
The exhaust gas sensor called a lambda type as a popular name to feed-back and control the air fuel ratio of the internal combustion engine, is widely practically used. For example, this lambda type exhaust gas sensor is arranged before and after a catalyst arranged in an exhaust pipe path, and generates a detecting voltage Vs according to oxygen concentration within exhaust gas. This detecting voltage Vs is about 0.45 V in a theoretical air fuel ratio as a driving target. However, this detecting voltage Vs has characteristics in which this detecting voltage is rapidly raised in a rich state of fuel, and is converged to a value of 0.85 to 1.0 V, and is rapidly lowered in a lean state of the fuel and is converged to 0.1 to 0 V.
However, it is general that the detecting voltage in a low temperature and non-activating state of this exhaust gas sensor is small, and internal resistance reaches a level of several MΩ although the internal resistance is several ten KΩ in an activating temperature state. Various techniques are proposed and practically used to detect the breakdown and the wiring abnormality of such an exhaust gas sensor.
For example, in accordance with JP-A-05-107299 “Connecting circuit for lambda sensor and inspecting method of this circuit”, an offset voltage is applied to a ground lead (negative line) of the lambda sensor for detecting the air fuel ratio. The detecting voltage generated between a positive signal line and the negative line of the lambda sensor is amplified. A value provided by adding the above offset voltage is digitally converted and is then inputted to a microprocessor. If the detecting voltage inputted to the microprocessor is a value less than the above offset voltage, it is judged as a ground state in which the positive signal line comes in short circuit contact with a ground circuit. Further, it is detected as a ground abnormality at a disconnection abnormality time of the signal line by connecting a pull-down resistor to the positive signal line.
Further, in accordance with JP-A-2005-171898 “breakdown diagnosing device and breakdown diagnosing method of air fuel ratio sensor”, the offset voltage is applied to the negative line of the air fuel ratio sensor. An electric potential of the positive signal line of the air fuel ratio sensor with respect to the ground circuit is digitally converted and is then inputted to the microprocessor. If the detecting voltage inputted to the microprocessor is a value less than the above offset voltage, it is judged as a ground state in which the positive signal line comes in short circuit contact with the ground circuit.
Further, the pull-down resistor is connected to the positive signal line, and it is possible to discriminate whether it is a ground abnormality or a disconnection abnormality by temporarily connecting a pull-up resistor.
Explanation of Problem of Former Technique
In accordance with the above JP-A-05-107299 and JP-A-2005-171898, the offset voltage is applied to the negative line of the exhaust gas sensor. Thus, if the exhaust gas sensor and its wiring are normal, an abnormality judgment relative to the positive signal line is made by logic in which no electric potential of the positive signal line becomes the above offset voltage or less.
However, a ground abnormality of the ground line (negative line) applying the offset voltage thereto, and a sky short circuit abnormality for coming in mixing contact with an electric power source line are not detected. Accordingly, there is a defect in which no abnormality judgment is synthetically made.